1. Field of the Invention
The present invention relates to improved microporous electroconducting materials and cathodes/electrolytic cells comprised thereof, in particular cathodes/electrolytic cells for the electrolysis of alkali metal halide solutions.
This invention also relates to a process for the preparation of such novel electroconductive substrates.
The present invention further relates to composites of such cathodes that include a diaphragm and the electrolysis of alkali metal halide solutions utilizing same.
2. Description of the Prior Art
Chlorine and sodium hydroxide (caustic soda) are conventionally produced via electrolysis of aqueous solutions of sodium chloride (chlor-alkali units).
Materials suitable for use as cathodes in such NaCl electrolytic cells must possess certain properties, i.e., low electrical resistivity compatible with conducting the electrolysis at an acceptable energy level, small thickness (about 0.1 to 10 mm), and a large surface area which can be up to several square meters.
In addition, it must be possible to produce these materials by deposition onto a rigid structure having a great number of large diameter openings.
Such electroconductive materials are typically produced by vacuum filtration of a suspension of fibers and binders therefor.
The properties of the material depend on a number of parameters, in particular the nature and concentration of the fibrous matter in suspension, surfactants, porogens and other additives.
A fibrous material is known to this art which combines such properties and which comprises a mixture of conducting and nonconducting fibers, as described in EP-A-0,319,517. This material comprises a mixture of asbestos and carbon fibers, with the carbon fibers imparting the electroconductivity, and the asbestos fibers fixing the binders during filtration.
Also, a number of improvements have been made to this material and to the preparative technique for the manufacture thereof.
Thus, EP-A-0,214,066 describes materials comprising carbon fibers having a monodisperse distribution of lengths and exhibiting considerably improved properties and quality, namely, a far better performance/thickness ratio.
EP-A-0,296,076 describes an electroactive material comprising an electrocatalytic agent which is uniformly distributed throughout its mass, said agent being selected from among Raney metals and Raney alloys and from which a major portion of the readily removable metal(s) has indeed been eliminated.
All of the cathode elements described ensure considerable current distribution and are suited for use in electrolytic cells that also comprise a membrane or diaphragm separating the anode and cathode compartments.
Nonetheless, the quality of the aforedescribed cathode elements, as well as others known to this art, is not completely satisfactory because of the requirement for asbestos fibers. In addition to the risks to human health attributed to the handling of this dangerous substance, a number of problems are associated with the inherent chemical instability of asbestos, such as too short a useful life of electrolytic cells containing such cathodes and difficulties in modifying the operating conditions of electrolysis, for example when increasing the electric power and/or the concentration of alkali metal hydroxide.
EP-A-0,222,671, assigned to the assignee hereof, describes, in one example, production of an asbestos-free precathode from a suspension which contains only conducting carbon fibers. The composition of the suspension, however, does not permit an industrial coating to be produced having a porosity which is both fine and regular and which firmly adheres to the cathode.